MARYLAND GEOLOGICAL SURVEY 343 



adopted. According to the researches of E. 0. Ulrich as yet unpublished, 

 Green's casts of Isotelus stegops, the originals of which came from New- 

 port, Kentucky, show no good differences from the Eden shale species of 

 that region. 



Compared with Isotelus maximus Locke, which 7. stegops most closely 

 resembles, the Eden form has the eye further forward, smaller spines, and 

 the flattened border, especially of the pygidium, less distinct. 



Occurrence. MARTINSBURG SHALE (Eden division). Jordans Knob, 

 one and one-half miles northeast of Fort Loudon, and other localities in 

 Pennsylvania. 



Collection. U. S. National Museum. 



ISOTELUS MEGISTOS Locke 



Plate LVIII, Figs. 10, 11 



Isotelus megistos Locke, 1842, Amer. Jour. Scl., vol. xlii, p. 366, pi. iii, fig. 9. 

 Asaphus (Isotelus) megistos Meek, 1873, Geol. Surv. Ohio, Pal., vol. I, 

 p. 159, pi. xiv, fig. 13. 



Description. Under this name and also those of Isotelus maximus 

 Locke and 7. gigas DeKay, a number of distinct species ranging through 

 the Mohawkian and Cincinnatian have undoubtedly been confused, with 

 the result that these names have little stratigraphic significance. The 

 discrimination of these species has been undertaken by E. 0. Ulrich, 

 whose work upon them is still in manuscript form. He has determined 

 that the fragments found in the Fairview deposits of Maryland and 

 Pennsylvania are identical specifically with the types of Locke's Isotelus 

 megistos and also with the specimen illustrated later by Meek as Asaphus 

 (Isotelus) megistos. 



Formerly the separation of these species was based upon the presence 

 or absence of the genal spine, but it is now known that each species 

 contains spinuous and aspinous forms, the difference between the two 

 being presumably that of sex. The aspinous ( ? female) forms of these 

 several species are quite difficult to distinguish from each other, but the 

 spinous examples show good characters of differentiation. Thus, in the 

 case of 7. maximus and 7. megistos, long considered synonymous, the free 



